TMR/GMR amplifier with input current compensation

ABSTRACT

An amplifier system with feedback current cancellation comprises an amplifier with at least one stage, a feedback network, first and second replica circuits, a buffer, second and third resistances, an operational amplifier (op-amp), a transistor, and a current mirror. The feedback network includes a first resistance that communicates with an input and an output of the amplifier. The first and second replica circuits approximately replicate DC characteristics of the output and the input of the amplifier, respectively. An output of the op-amp communicates with a control terminal of the transistor. The current mirror provides a current at the input of the amplifier that is proportional to a second current flowing through the transistor. The buffer communicates with the first replica circuit. The second resistance communicates with an output of the buffer. The third resistance communicates with the second resistance and with the second replica circuit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/929,211 filed on Aug. 30, 2004. The disclosure of the aboveapplication is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to amplifiers, and more particularly tofeedback error current cancellation in a closed-loop amplifier.

BACKGROUND OF THE INVENTION

Referring to FIG. 1, an exemplary magnetic storage system 100 such as ahard disk drive is shown. A buffer 102 stores data that is associatedwith control of the hard disk drive. The buffer 102 may employ SDRAM orother types of low latency memory. A processor 104 performs processingthat is related to the operation of the hard disk drive. A hard diskcontroller (HDC) 106 communicates with the buffer 102, the processor104, a host 108 via an I/O channel 110, a spindle/voice coil motor (VCM)driver 112, and a read/write channel circuit 114.

During a write operation, the read/write channel circuit 114 encodes thedata to be written onto the storage medium. The read/write channelcircuit 114 processes the signal for reliability and may include, forexample, error checking and correcting coding (ECC) and run lengthlimited coding (RLL). During read operations, the read/write channelcircuit 114 converts an analog output from the medium to a digitalsignal. The converted signal is then detected and decoded by knowntechniques to recover the data written on the hard disk drive.

One or more hard drive platters 116 include a magnetic coating thatstores magnetic fields. The platters 116 are rotated by a spindle motorthat is schematically shown at 118. Generally, the spindle motor 118rotates the hard drive platters 116 at a fixed speed during read/writeoperations. One or more read/write arm(s) 120 move relative to theplatters 116 to read and/or write data to/from the hard drive platters116. The spindle/VCM driver 112 controls the spindle motor 118, whichrotates the platters 116. The spindle/VCM driver 112 also generatescontrol signals that position the read/write arm 120, for example usinga voice coil actuator, a stepper motor, or any other suitable actuator.

A read/write device 122 is located near a distal end of the read/writearm 120. The read/write device 122 includes a write element such as aninductor that generates a magnetic field. The read/write device 122 alsoincludes a read element (such as a magneto-resistive (MR) sensor) thatsenses the magnetic fields on the platter 116. A preamplifier (preamp)circuit 124 amplifies analog read/write signals. When reading data, thepreamp circuit 124 amplifies low level signals from the read element andoutputs the amplified signal to the read/write channel circuit 114.While writing data, a write current that flows through the write elementof the read/write channel circuit 114 is switched to produce a magneticfield having a positive or negative polarity. The positive or negativepolarity is stored by the hard drive platter 116 to represent data.

Referring now to FIG. 2, an exemplary circuit 140 is presented thatamplifies a signal across a variable resistance 142. This circuit couldbe used in the preamplifier of a magnetic storage system where thevariable resistance is the MR sensor. A first current source 144communicates with a supply potential 146. A second current source 148communicates with a ground potential 150. The first and second currentsources 144 and 148 communicate with a first terminal and an oppositeterminal of a variable resistance 142, respectively. The first terminalof a variable resistance 142 communicates with a first terminal of afirst capacitance 152. The opposite terminal of the variable resistance142 communicates with a first terminal of a second capacitance 154.

A second terminal of the first capacitance 152 communicates with anoninverting input of a differential operational amplifier (op-amp 156).A second terminal of the second capacitance 154 communicates with aninverting terminal of the op-amp 156. The first terminal of the firstcapacitance 152 communicates with a first terminal of a first resistance158. An opposite terminal of the first resistance 158 communicates witha first terminal of a third capacitance 160. A second terminal of thethird capacitance 160 communicates with a noninverting output of theop-amp 156. The first terminal of the second capacitance 154communicates with a first terminal of a second resistance 162. Anopposite terminal of the second resistance 162 communicates with a firstterminal of a fourth capacitance 164. A second terminal of the fourthcapacitance 164 communicates with an inverting output of the op-amp 156.

Traditionally, the third and fourth capacitances 160 and 164 have beenincluded to block DC current from flowing through the first and secondresistances 158 and 162, respectively. These error currents would thenflow through the variable resistance 142, causing its bias current todiffer from what the first and second current sources 144 and 148establish. Also, the current pulled from the output of the op-amp 156 bythe first and second resistances 158 and 162 would affect the op-amp'sperformance. However, the third and fourth capacitances 160 and 164create low-pass filters that interact with the high-pass filters createdby the first and second capacitances 152 and 154, making the overallfrequency response of the circuit difficult to design.

SUMMARY OF THE INVENTION

An amplifier system with feedback current cancellation in someembodiments comprises an amplifier with an input, an output, and atleast one stage. A feedback network communicates with the input andoutput of the amplifier. A feedback current cancellation module providesa first current at the input of the amplifier that substantially cancelsa second current provided at the input of the amplifier by the feedbacknetwork.

In other features, the feedback current cancellation module is biased bya standby network that operates on standby power when power to theamplifier is turned off. The standby network is a low power circuit thatapproximately replicates DC characteristics at the input and the outputof the amplifier.

In still other features, the feedback current cancellation moduleprovides a third current at the output of the amplifier thatsubstantially cancels a fourth current provided at the output of theamplifier by the feedback network. The feedback current cancellationmodule is biased by a standby network that operates on standby powerwhen power to the amplifier is turned off. The standby network is a lowpower circuit that approximately replicates DC characteristics at theinput and the output of the amplifier.

A sensor system comprises the amplifier system and further comprisescurrent source and a variable resistance that communicates with, and isbiased by, the current source. The input of the amplifier communicateswith a terminal of the variable resistance. A hard disk drive systemcomprises the sensor system. In other features, the amplifier isarranged in a differential mode or a single-ended mode. A DC blockingcapacitance communicates with the input of the amplifier.

An amplifier system with feedback current cancellation comprises anamplifier, a feedback network, first and second replica circuits, abuffer, a second resistance, a third resistance, an operationalamplifier (op-amp), a transistor, and a first current mirror. Theamplifier includes an input, an output, and at least one stage. Thefeedback network includes a first resistance having a first end thatcommunicates with the input of the amplifier and an opposite end thatcommunicates with the output of the amplifier.

First and second replica circuits approximately replicate DCcharacteristics of the output and the input of the amplifier,respectively. The buffer includes an input that communicates with thefirst replica circuit and an output. The second resistance includes afirst end that communicates with the output of the buffer and anopposite end. The third resistance includes a first end thatcommunicates with the opposite end of the second resistance and anopposite end that communicates with the second replica circuit.

The operational amplifier (op-amp) includes a first input thatcommunicates with the first end of the third resistance, a second inputthat communicates with the opposite end of the third resistance, and anoutput. The transistor includes a control terminal that communicateswith the output of the op-amp and a first terminal that communicateswith the opposite end of the second resistance. The first current mirrorprovides a first current at the input of the amplifier that isproportional to a second current flowing through the transistor.

In other features, the second resistance has a resistance value that isapproximately equal to a resistance value of the first resistance. Thefirst current is approximately equal to the second current. The firstand second replica circuits, the buffer, and the op-amp operate onstandby power when power to the amplifier is turned off.

In further features, a sensor system comprises the amplifier system andfurther comprises a first current source and a variable resistance thatcommunicates with, and is biased by, the first current source. The inputof the amplifier communicates with one end of the variable resistance. Ahard disk drive system comprises the sensor system.

In still other features, the second replica circuit comprises a secondcurrent source and a variable resistance. The second current sourceprovides current that is proportional to current provided by the firstcurrent source. The variable resistance communicates with, and is biasedby, the second current source, and has a resistance value that isproportional to a nominal resistance value of the variable resistance.

In other features, the amplifier system further comprises a secondcurrent mirror that provides a third current at the output of theamplifier that is approximately equal to the first current. A sensorsystem comprises the amplifier system and further comprises a firstcurrent source, and a variable resistance that communicates with, and isbiased by, the first current source.

The input of the amplifier communicates with one end of the variableresistance. In further features, a hard disk drive system comprises thesensor system. The amplifier is arranged in a differential mode or asingle-ended mode. The amplifier system further comprises a DC blockingcapacitance that communicates with the input of the amplifier.

An amplifying system with feedback current cancellation comprisesamplifying means, feedback means, buffering means, transconductingmeans, operational amplifying (op-amp) means, first and secondreplicating means, first current mirroring means, and second and thirdresisting means. The amplifying means is for amplifying a signal at aninput and presenting an amplified version of the signal at an output andincludes at least one stage.

The feedback means is for providing a feedback signal and includes firstresisting means for providing resistance between the input and theoutput of the amplifying means. The buffering means is for buffering aninput and presenting a result at an output. The transconducting means isfor providing a first current at a first terminal based upon a voltageat a control terminal. The operational amplifying (op-amp) means is foramplifying a difference between first and second inputs and presenting aresult at an output.

The first input and the output communicate with the first terminal andthe control terminal of the transconducting means, respectively. Thefirst and second replicating means are for approximately replicating DCcharacteristics of the output and the input of the amplifying means,respectively. The first replicating means communicates with the input ofthe buffering means and the second replicating means communicates withthe second input of the op-amp means.

The first current mirroring means is for providing a second current atthe input of the amplifying means that is proportional to the firstcurrent. The second resisting means is for providing resistance betweenthe output of the buffering means and the first input of the op-ampmeans. The third resisting means is for providing resistance between theinputs of the op-amp means.

In other features, the second resisting means has a resistance valuethat is approximately equal to a resistance value of the first resistingmeans. The first current is approximately equal to the second current.The first and second replicating means, the buffering means, and theop-amp means operate on standby power when power to the amplifying meansis turned off.

In further features, a sensing system comprises the amplifying systemand further comprises first current sourcing means for providing abiasing current; and variable resisting means for providing a voltagesignal when biased with the biasing current. The input of the amplifyingmeans communicates with one end of the variable resisting means.

In still other features, a hard disk drive system comprises the sensingsystem. The second replicating means comprises second current sourcingmeans for providing a replica biasing current that is proportional tothe biasing current; and fourth resisting means for providing a replicavoltage signal when biased with the replica biasing current and that hasa resistance value that is proportional to a nominal resistance value ofthe variable resisting means.

In other features, the amplifying system further comprises secondcurrent mirroring means for providing a third current at the output ofthe amplifying means that is approximately equal to the second current.A sensing system comprises the amplifying system and further comprisesfirst current sourcing means for providing a biasing current; andvariable resisting means for providing a voltage signal when biased withthe biasing current. The input of the amplifying means communicates withone end of the variable resisting means.

In further features, a hard disk drive system comprises the sensingsystem. The amplifying means is arranged in a differential mode. Theamplifying means is arranged in a single-ended mode. The amplifyingsystem further comprises DC blocking means for blocking DC signals fromentering the input of the amplifying means.

Further areas of applicability of the present invention will becomeapparent from the detailed description provided hereinafter. It shouldbe understood that the detailed description and specific examples, whileindicating the preferred embodiment of the invention, are intended forpurposes of illustration only and are not intended to limit the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 is an exemplary hard disk drive environment according to theprior art;

FIG. 2 is an exemplary circuit which amplifies a signal across avariable resistance, according to the prior art;

FIG. 3 is a block diagram of an exemplary amplifier incorporating afeedback error current cancellation system;

FIG. 4 is an electrical schematic of an exemplary amplifier employing afeedback current cancellation system;

FIG. 5 is an electrical schematic of an exemplary amplifier includingfeedback error current cancellation at both terminals of the amplifier;

FIG. 6 is an electrical schematic of an exemplary amplifier includingfeedback error current cancellation at both terminals of the amplifierand replica biasing;

FIG. 7 is a hard drive employing a preamplifier according to theprinciples of the present invention; and

FIG. 8 is an electrical schematic of an exemplary amplifier employing analternate feedback current compensation device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the preferred embodiments is merelyexemplary in nature and is in no way intended to limit the invention,its application, or uses. For purposes of clarity, the same referencenumbers will be used in the drawings to identify similar elements. Asused herein, the term module refers to an application specificintegrated circuit (ASIC), an electronic circuit, a processor (shared,dedicated, or group) and memory that execute one or more software orfirmware programs, a combinational logic circuit, and/or other suitablecomponents that provide the described functionality.

Referring now to FIG. 3, a block diagram of an exemplary amplifier 200incorporating a feedback error current cancellation system is depicted.An input device 202 communicates a signal to an input of an amplifier204. The input of the amplifier 204 also communicates with a firstterminal of a feedback network 206 and a first terminal of a feedbackcurrent cancellation module 208. An output of the amplifier 204communicates with an output device 210. The output of the amplifier 204also communicates with a second terminal of the feedback network 206 anda second terminal of the feedback current cancellation module 208. Thefeedback network 206 draws current I_(ERR) into the second terminal. Thefeedback network 206 also communicates current equal to I_(ERR) out ofthe first terminal.

Without the feedback current cancellation module 208, the currentI_(ERR) going into the second terminal of the feedback network 206 wouldbe drawn from the output of the amplifier 204. The current leaving thefirst terminal of the feedback network 206, which is equal to I_(ERR)would flow into the input device 202. The feedback current cancellationmodule 208 provides current that is opposite to I_(ERR) to the input ofthe amplifier 204. The current from the first terminal of the feedbackcurrent cancellation module 208 and from the first terminal of thefeedback network 206 cancel at the input of the amplifier 204. Thus nocurrent passes back into the input device 202. In other words, I_(ERR)flows from the first terminal of the feedback network 206 into the firstterminal of the feedback current cancellation module 208. Optionally,the feedback current cancellation module 208 can also provide currentI_(ERR) to the output of the amplifier 204. I_(ERR) would then flow fromthe second terminal of the feedback current cancellation module 208 tothe second terminal of the feedback network 206. Therefore, the feedbacknetwork 206 would draw no current from the output of the amplifier 204.

Skilled artisans will recognize that the currents referenced in thedescription of FIG. 3 may be negative as well as positive. For example,if I_(ERR) is negative, the feedback network 206 draws a current intoits first terminal and provides a current out of its second terminal. Inthis case, the feedback cancellation module 208 would provide current atthe first terminal of the feedback network 206 such that the currentwould flow directly from the feedback cancellation module 208 to thefeedback network 206 without drawing current from the input device 202.

Referring now to FIG. 4, an exemplary electrical schematic of anamplifier 230 employing a feedback current cancellation system is shown.For purposes of illustration, a single-ended implementation is shown.Skilled artisans will recognize that the same techniques can be appliedequally to a differential amplifier. A first current source 232communicates with a supply potential 234. The first current source 232also communicates with a first terminal of a variable resistance 236. Anopposite terminal of the variable resistance 236 communicates with aground potential 238. The first terminal of the variable resistance 236communicates with a first terminal of a first capacitance 240. A secondterminal of the first capacitance 240 communicates with an input of asingle-ended amplifier 242.

Skilled artisans will recognize that the single-ended amplifier 242 mayinclude multiple stages. An output of the single-ended amplifier 242communicates with a first terminal of a first resistance 244. Anopposite terminal of the first resistance 244 communicates with thefirst terminal of the first capacitance 240. The output of thesingle-ended amplifier 242 communicates with a first terminal of asecond resistance 246. An opposite terminal of the second resistance 246communicates with a noninverting input of an operational amplifier(op-amp) 248. The noninverting input of the op-amp 248 communicates witha first terminal of a third resistance 250. An opposite terminal of thethird resistance 250 communicates with an output of the op-amp 248. Afirst terminal of a fourth resistance 252 communicates with the outputof the op-amp 248. An opposite terminal of the fourth resistancecommunicates with an inverting input of the op-amp. The oppositeterminal of the fourth resistance 252 also communicates with the firstterminal of the first capacitance 240.

The voltage at the output of the single-ended amplifier 242 will bereferred to as V_(O). The voltage at the first terminal of the variableresistance 236 will be referred to as V_(I). In one embodiment, thefirst and fourth resistances 244 and 252 are both equal to the valueR_(F), and the second and third resistances 246 and 250 are both equalto the value R_(OP). Assuming that V_(O) is greater than V_(I), an errorcurrent, I_(ERR), flowing through the first resistance 244 from theoutput of the single-ended amplifier 242 to the first terminal of thevariable resistance 236 is equal to (V_(O)−V_(I))/R_(F). The voltage atthe output of the op-amp 248 is equal to 2V_(I)−V_(O). The currentflowing through the fourth resistance 252 between the output of theop-amp 248 and the first terminal of the variable resistance 236 is thenequal to (V_(I)−V_(O))/R_(F). This is the opposite of I_(ERR), so thatthe currents through the first and fourth resistances 244 and 252 cancelat the first terminal of the variable resistance 236. Therefore, thebias current flowing through the variable resistance 236 will beestablished only by the first current source 232.

If V_(O) is less than V_(I), the current (V_(O)−V_(I))/R_(F) isnegative, meaning that I_(ERR) physically flows from the first terminalof the variable resistance 236 to the output of the single-endedamplifier 242. The current flowing through the fourth resistance 252,(V_(I)−V_(O))/R_(F), is then positive and thus still cancels I_(ERR) atthe first terminal of the variable resistance 236.

Referring now to FIG. 5, an electrical schematic of an exemplaryamplifier 270 including feedback error current cancellation at bothterminals of the amplifier is portrayed. A first current source 272communicates with a supply potential 274. The first current source 272also communicates with a first terminal of a variable resistance 276. Anopposite terminal of the variable resistance 276 communicates with aground potential 278. The first terminal of the variable resistance 276communicates with a first terminal of a first capacitance 280. A secondterminal of the first capacitance 280 communicates with an input of asingle-ended amplifier 282. Skilled artisans will recognize that thesingle-ended amplifier 282 may include multiple stages. An output of thesingle-ended amplifier 282 communicates with a first terminal of a firstresistance 284. An opposite terminal of the first resistance 284communicates with the first terminal of the first capacitance 280. Theoutput of the single-ended amplifier 282 communicates with anoninverting input of a first op-amp 286.

This circuit includes first, second, third, fourth, fifth, and sixthtransistors 288, 290, 292, 294, 296, and 298, respectively. In thisimplementation the first, second, third, fourth, fifth, and sixthtransistors 288, 290, 292, 294, 296, and 298 are metal-oxidesemiconductor field-effect transistors (MOSFETs) that have gates,sources, and drains, although other transistor types may be used. Aninverting input of the first op-amp 286 communicates with a source (orsecond terminal) of the first transistor 288. An output of the firstop-amp 286 communicates with a gate (or control terminal) of the firsttransistor 288. The source of the first transistor 288 communicates witha first terminal of a second resistance 300. An opposite terminal of thesecond resistance communicates with an inverting input of a secondop-amp 302. The opposite terminal of the second resistance 300 alsocommunicates with a source of the second transistor 290. A gate of thesecond transistor 290 communicates with an output of the second op-amp302. A drain (or first terminal) of the second transistor 290communicates with a drain of the third transistor 292.

The drain of the third transistor 292 communicates with a gate of thethird transistor 292. A source of the third transistor 292 communicateswith the ground potential 278. The gate of the third transistor 292communicates with a gate of the fourth transistor 294. A source of thefourth transistor 294 communicates with the ground potential 278. Adrain of the fourth transistor 294 communicates with a noninvertinginput of the second op-amp 302. The drain of the fourth transistor 294also communicates with the first terminal of the variable resistance276. A drain of the first transistor 288 communicates with a drain ofthe fifth transistor 296. The drain of the fifth transistor 296communicates with a gate of the fifth transistor 296. The gate of thefifth transistor 296 communicates with a gate of the sixth transistor298. A drain of the sixth transistor 298 communicates with the output ofthe single-ended amplifier 282. A source of the fifth transistor 296 anda source of the sixth transistor 298 both communicate with the supplypotential 274.

The voltage at the output of the single-ended amplifier 282 is referredto as V_(O). The voltage at the first terminal of the variableresistance 276 is referred to as V_(I). The first resistance 284 and thesecond resistance 300 are both equal to the value R_(F). The voltage atthe first terminal of the second resistance 300 will be equal to V_(O).The voltage at the opposite terminal of the second resistance 300 willbe equal to V_(I). The current through the second resistance 300 willthen be equal to (V_(O)−V_(I))/R_(F). This current, referred to asI_(OFF), also passes through the fifth and the third transistors 296 and292. The fifth and the sixth transistors 296 and 298 form current mirrorso that I_(OFF) also passes through the sixth transistor 298.

The current flowing through the first resistance 284, I_(ERR), is equalto (V_(O)−V_(I))/R_(F). I_(ERR) and I_(OFF) are thus equal, so that thecurrent flowing through the first resistance 284 is drawn solely fromthe drain of the sixth transistor 298 and not from the output of thesingle-ended amplifier 282. The amount of current drawn by the feedbackresistor 284 can be significant, and I_(OFF) prevents that feedbackcurrent from altering the performance of the single-ended amplifier 282.The third and fourth transistors 292 and 294 also form current mirror,causing I_(OFF) to flow through the fourth transistor 294. The currentI_(ERR), being equal to I_(OFF), will flow directly from the oppositeterminal of the first resistance 284 to the drain of the fourthtransistor 294 and will not pass through the variable resistance 276.Therefore, the bias current through the variable resistance 276 remainsat the level set by the first current source 272.

FIG. 5 depicts a situation where V_(O) is greater than V_(I). Generally,the circuit designer will know whether V_(O) will be greater than orless than V_(I). Skilled artisans will recognize that for FIG. 5 to beadapted to a situation where V_(O) is less than V_(I), connections atthe input and output of the single-ended amplifier 282 may be reversed.That is, the drain of the fourth transistor 294 and the noninvertinginput of the second op-amp 302 will communicate with the output of thesingle-ended amplifier 282, while the drain of the sixth transistor 298and the noninverting input of the first op-amp 286 will communicate withthe first terminal of the first capacitance 280.

Skilled artisans will also recognize that the resistance value of thesecond resistance 300 can be scaled with respect to the resistance valueof the first resistance 284. In this case, third and fourth transistors292 and 294 may be scaled appropriately, as well as fifth and sixthtransistors 296 and 298.

Referring now to FIG. 6, an electrical schematic of an exemplaryamplifier 320 including feedback error current cancellation at bothterminals of the amplifier and replica biasing for fast recovery isportrayed. A first current source 322 communicates with a first supplypotential 324. The first current source 322 also communicates with afirst terminal of a variable resistance 326. An opposite terminal of thevariable resistance 326 communicates with a ground potential 328. Thefirst terminal of the variable resistance 326 communicates with a firstterminal of a first capacitance 330. A second terminal of the firstcapacitance 330 communicates with an input of a single-ended amplifier332. Skilled artisans will recognize that the single-ended amplifier 332may include multiple stages. An output of the single-ended amplifier 332communicates with a first terminal of a first resistance 334. Anopposite terminal of the first resistance 334 communicates with thefirst terminal of the first capacitance 330.

This circuit includes first, second, third, fourth, fifth, and sixthtransistors 336, 338, 340, 342, 344, and 346, respectively. In thisimplementation, the first, second, third, fourth, fifth, and sixthtransistors 336, 338, 340, 342, 344, and 346 are metal-oxidesemiconductor field-effect transistors (MOSFETs) that have gates,sources, and drains, although other transistor types may be used. Aninverting input of a first op-amp 348 communicates with a source (orsecond terminal) of the first transistor 336. An output of the firstop-amp 348 communicates with a gate (or control terminal) of the firsttransistor 336. The source of the first transistor 336 communicates witha first terminal of a second resistance 350. An opposite terminal of thesecond resistance 350 communicates with an inverting input of a secondop-amp 352. The opposite terminal of the second resistance 350 alsocommunicates with a source of the second transistor 338. A gate of thesecond transistor 338 communicates with an output of the second op-amp352. A drain (or first terminal) of the second transistor 338communicates with a drain of the third transistor 340. The drain of thethird transistor 340 communicates with a gate of the third transistor340. A source of the third transistor 340 communicates with the groundpotential 328. The gate of the third transistor 340 communicates with afirst terminal of a first switch 354.

A second terminal of the first switch 354 communicates with a gate ofthe fourth transistor 342. A source of the fourth transistor 342communicates with the ground potential 328. A drain of the fourthtransistor 342 communicates with the first terminal of the variableresistance 326. A drain of the first transistor 336 communicates with adrain of the fifth transistor 344. The drain of the fifth transistor 344communicates with a gate of the fifth transistor 344. A source of thefifth transistor communicates with a second supply potential 356. Thegate of the fifth transistor 344 communicates with a first terminal of asecond switch 358. A second terminal of the second switch 358communicates with a gate of the sixth transistor 346. A drain of thesixth transistor 346 communicates with the output of the single-endedamplifier 332. A source of the sixth transistor 346 communicates withthe first supply potential 324.

A scaled-down replica 360 of the last stage of the single-endedamplifier 332 communicates with a noninverting input of the first op-amp348. The replica 360 requires less current, and thus less power, thanthe last stage of the single-ended amplifier 332, but has the same DCoperating characteristics. In other words, the replica 360 will outputthe same DC voltage that the single-ended amplifier 332 does.

A second current source 362 communicates with the second supplypotential 356. The second current source 362 communicates with a firstterminal of a third resistance 364. An opposite terminal of the thirdresistance 364 communicates with the ground potential 328. The firstterminal of the third resistance 364 communicates with a noninvertinginput of the second op-amp 352.

The voltage at the output of the single-ended amplifier 332 is referredto as V_(O). The voltage at the first terminal of the variableresistance 326 is referred to as V_(I). The first resistance 334 and thesecond resistance 350 are both equal to the value R_(F). The currentprovided by the second current source 362 is equal to the currentprovided by the first current source 322 divided by a constant K. Thevalue of the third resistance 364 is equal to the nominal value of thevariable resistance 326 multiplied by the constant K. The voltage at thefirst terminal of the third resistance 364 will be equal to the nominalvalue of V_(I) because the second current source 362 and the thirdresistance 364 are scaled in opposite directions by the same factor K.The voltage at the noninverting input of the first op-amp 348 will beequal to the nominal value of V_(O) because the DC value at the outputof the single-ended amplifier 332 will be the same as that of itsscaled-down last stage. The voltage at the first terminal of the secondresistance 350 will then be equal to V_(O). The voltage at the oppositeterminal of the second resistance 350 will then be equal to V_(I). Thecurrent through the second resistance 350 will then be equal to(V_(O)−V_(I))/R_(F). This current, referred to as I_(OFF), also passesthrough the fifth and third transistors 344 and 340.

In normal operation, the fifth and sixth transistors 344 and 346 formcurrent mirror so that I_(OFF) also passes through the sixth transistor346. The current flowing through the first resistance 334, I_(ERR), isequal to (V_(O)−V_(I))/R_(F). I_(ERR) and I_(OFF) are thus equal, sothat the current through the first resistance 334 is drawn solely fromthe drain of the sixth transistor 346 and not from the output of thesingle-ended amplifier 332. This prevents feedback current from alteringthe performance of the single-ended amplifier 332. The third and fourthtransistors 340 and 342 also form current mirror, causing I_(OFF) toflow through the fourth transistor 342. The current I_(ERR), being equalto I_(OFF), will flow directly from the opposite terminal of the firstresistance 334 to the drain of the fourth transistor 342 and will notpass through the variable resistance 326. Therefore, the bias currentthrough the variable resistance 326 remains at the level set by thefirst current source 322.

The second supply potential 356 is energized so long as the amplifier ispowered on. During normal operation, the first and second switches 354and 358 are both in a closed, or conducting, position. Additionally, thefirst supply potential 324 is on, i.e. at the same potential as thesecond supply potential 356. If the amplifier is to be put into a sleep,or low power, mode, the first and second potentials could both be turnedoff. However, when they would be turned back on, the circuit would takea significant amount of time to recover, or stabilize. The currentimplementation causes only the first supply potential 324 to be poweredoff in order to put the circuit into sleep mode. When the first supplypotential 324 is turned off, the first and second switches 354 and 358are opened to disconnect the current mirrors from the single-endedamplifier 332. The second current source 362 and last stage replica 360are scaled to consume less power. The replicated voltages at the inputsto the first and second op-amps 348 and 352 allow the current I_(OFF) tobe maintained when the first supply potential 324 is off. When the firstsupply potential 324 is turned on, the current I_(OFF) is ready to allowthe amplifier to recover from sleep mode quickly.

FIG. 6 depicts a situation where V_(O) is greater than V_(I). Generally,the circuit designer will know whether V_(O) will be greater than orless than V_(I). Skilled artisans will recognize that for FIG. 6 to beadapted to a situation where V_(O) is less than V_(I), input and outputconnections may be reversed. That is, the noninverting input of thefirst op-amp 348 communicates with the first terminal of the thirdresistance 364, the noninverting input of the second op-amp 352communicates with the replica 360, the drain of the sixth transistor 346communicates with the first terminal of the first capacitance 330, andthe drain of the fourth transistor 342 communicates with the output ofthe single-ended amplifier 332.

Skilled artisans will also recognize that the resistance value of thesecond resistance 350 can be scaled with respect to the resistance valueof the first resistance 334. In this case, third and fourth transistors340 and 342 may be scaled appropriately, as well as fifth and sixthtransistors 344 and 346.

Referring now to FIG. 7, a hard disk drive environment 380 similar tothat of FIG. 1 is presented, with the preamplifier 124 being replaced bya new preamplifier 390 according to the principles of the presentinvention. The new preamplifier 390 is not susceptible to feedback errorcurrents and yet does not have a complex frequency response that makesits design difficult. Further, addition of feedback current compensationdoes not necessarily increase the time the new preamplifier 390 takes torecover from sleep mode.

Referring now to FIG. 8, another circuit configuration that employsreplica biasing is shown. A last stage 400 communicates with a buffer402, which has an output that communicates with one end of a resistanceR_(F). An opposite end of the resistance R_(F) communicates with anon-inverting input of an opamp 410, one end of a sense resistanceR_(Sense) and a first terminal of a transistor M1. An opposite end ofthe sense resistance R_(Sense) communicates with current source 420 andone end of a resistance 424. A sense current I_(Sense) flows across thesense resistance R_(Sense) to generate a sense voltage across the inputsof the opamp 410. An output of the opamp 410 communicates with acapacitance C and with control inputs of the transistor M1, a transistor430 and a transistor 440 (through switch block SW₂). The switch blockSW₂ includes a first switch 442 that connects the control terminals ofthe transistors 430 and 440 and a second switch 443 that shorts thecontrol terminals to ground.

A first terminal of transistor 430 communicates with a first terminaland a control terminal of a transistor 444. The control terminal oftransistor 444 communicates with a control terminal of a transistor 448via switch block SW₁. The switch block SW₁ includes a first switch 452that connects the control terminals of the transistors 444 and 448 and asecond switch 453 that connects the control terminals to secondterminals thereof.

A first terminal of transistor 440 communicates with current source 460,a resistance 462, one end of resistance 464 and an input of amplifier468. An output of the buffer 468 communicates with an opposite end ofthe resistance 464 and with the first terminal of the transistor 448. Ascan be seen in FIG. 8, the error current I_(ERR) flows as shown. Thecircuit in FIG. 8 operates in a manner that is similar to the operationof the circuit in FIG. 6, which also uses replica biasing. Buffer 402prevents loading at a last stage replica circuit. When I_(Sense)≈0,I_(ERR)=IM₁. This condition will provide proper compensation. The switchblocks SW₁ and SW₂ are used for isolation during power down.

Those skilled in the art can now appreciate from the foregoingdescription that the broad teachings of the present invention can beimplemented in a variety of forms. Therefore, while this invention hasbeen described in connection with particular examples thereof, the truescope of the invention should not be so limited since othermodifications will become apparent to the skilled practitioner upon astudy of the drawings, the specification and the following claims.

1. An amplifier system with feedback current cancellation, comprising:an amplifier that includes an input, an output, and at least one stage;a feedback network that includes a first resistance having a first endthat communicates with the input of the amplifier and an opposite endthat communicates with the output of the amplifier; first and secondreplica circuits that approximately replicate DC characteristics of theoutput and the input of the amplifier, respectively; a buffer thatincludes an input that communicates with the first replica circuit andan output; a second resistance that includes a first end thatcommunicates with the output of the buffer and an opposite end; a thirdresistance that includes a first end that communicates with the oppositeend of the second resistance and an opposite end that communicates withthe second replica circuit; an operational amplifier (op-amp) thatincludes a first input that communicates with the first end of the thirdresistance, a second input that communicates with the opposite end ofthe third resistance, and an output; a transistor that includes acontrol terminal that communicates with the output of the op-amp and afirst terminal that communicates with the opposite end of the secondresistance; and a first current mirror that provides a first current atthe input of the amplifier that is proportional to a second currentflowing through the transistor.
 2. The amplifier system of claim 1wherein the second resistance has a resistance value that isapproximately equal to a resistance value of the first resistance, andwherein the first current is approximately equal to the second current.3. The amplifier system of claim 1 wherein the first and second replicacircuits, the buffer, and the op-amp operate on standby power when powerto the amplifier is turned off.
 4. A sensor system comprising theamplifier system of claim 1 and further comprising: a first currentsource; and a variable resistance that communicates with, and is biasedby, the first current source, wherein the input of the amplifiercommunicates with one end of the variable resistance.
 5. A hard diskdrive system comprising the sensor system of claim
 4. 6. The sensorsystem of claim 4 wherein the second replica circuit comprises: a secondcurrent source that provides current that is proportional to currentprovided by the first current source; and a fourth resistance thatcommunicates with, and is biased by, the second current source, and thathas a resistance value that is proportional to a nominal resistancevalue of the variable resistance.
 7. The amplifier system of claim 1further comprising a second current mirror that provides a third currentat the output of the amplifier that is approximately equal to the firstcurrent.
 8. A sensor system comprising the amplifier system of claim 7and further comprising: a first current source; and a variableresistance that communicates with, and is biased by, the first currentsource, wherein the input of the amplifier communicates with one end ofthe variable resistance.
 9. A hard disk drive system comprising thesensor system of claim
 8. 10. The amplifier system of claim 1 whereinthe amplifier is arranged in a differential mode.
 11. The amplifiersystem of claim 1 wherein the amplifier is arranged in a single-endedmode.
 12. The amplifier system of claim 1 further comprising a DCblocking capacitance that communicates with the input of the amplifier.